Transient Synovitis

Transient synovitis is a benign, self-limiting inflammation of the hip synovium and the most common cause of hip pain in children. The exam focus is almost entirely on distinguishing it from septic arthritis, an orthopaedic emergency that must be considered in every irritable hip. You must demonstrate a safe, logical approach using Kocher criteria; missing septic arthritis is a critical fail.

Who. The peak age is 3-8 years, most commonly 4-6, with a slight male predominance (1.5:1). It may follow a viral illness (winter/spring).

Natural history. Most children improve over several days and recover within roughly 1-2 weeks with rest and analgesia, and the prognosis is excellent when the course is typical and alternative pathology has been excluded. 5-15% may have recurrent episodes, and each recurrence still requires a fresh safety assessment.

Persistence is a red flag. Early Perthes disease, infection or another diagnosis may initially mimic synovitis. Failure to improve, recurrent symptoms, fever, night pain or inability to bear weight mandates reassessment rather than continued reassurance, with infection, Perthes disease, JIA and malignancy reconsidered.

Cause. The exact cause is unknown. It is widely accepted to be post-viral, often following an upper respiratory tract infection 1-2 weeks earlier, and post-infectious, an immune-mediated response to recent infection. It is non-bacterial and the joint fluid is sterile; a preceding viral illness is supportive but not diagnostic.

Why the hip hurts. Synovial inflammation produces a joint effusion, the effusion distends the capsule, and capsular distension causes pain and limitation of movement. The hip capsule is strong and non-compliant, so even small effusions cause significant pressure and pain.

Position of comfort. Children hold the leg in flexion, abduction and external rotation (FABER) to maximise capsular volume and reduce pressure.

Blood supply. The femoral head is supplied predominantly by retinacular branches of the medial circumflex femoral artery. Intracapsular pressure and inflammatory damage make this relevant in septic arthritis; transient synovitis does not justify assuming the same destructive pressure physiology.

There is no formal severity classification for transient synovitis. Kocher identified four predictors of septic arthritis in a selected irritable-hip cohort: a history of fever, non-weight-bearing, an ESR of at least 40 mm/hr and a WCC over 12,000 cells/mm³. They support pre-test assessment but are not a diagnosis or a management algorithm.

Reading the count. The predictors support pre-test assessment; they are not a diagnosis or management algorithm.

The limits of the score. Published percentages depend on the original, selected cohorts and should not be converted into automatic observation, aspiration or washout rules: no score proves or excludes infection. External validation has shown variable calibration, particularly with changing organism prevalence and younger Kingella infections.

Caird's addition. CRP over 20 mg/L was added in a small cohort already selected for aspiration. CRP is useful in assessment and serial monitoring, but the five-factor percentages are not universal and do not replace aspiration or clinical judgement.

Transient synovitis against septic arthritis. The predictors sit within a wider comparison of the two conditions.

History. Onset is acute or subacute, over hours to days. The child has hip or groin pain, which may refer to the thigh or knee, and either an antalgic limp or refusal to walk. Ask about a recent illness; the child usually appears well.

Examination. The child is usually afebrile or has a low-grade temperature (under 38.5°C). The findings:

• An antalgic limp, with a short stance phase
• Restricted internal rotation and abduction
• A log roll that may be irritable, but less so than in a septic hip
• Anterior joint-line tenderness

Bloods. The essential bloods, and how to read them:

• FBC - the WCC may be normal or mildly elevated; values over 12,000 contribute to Kocher risk assessment but are not diagnostic
• ESR - may be normal or mildly raised
• CRP - often low in a typical case, but age, timing and organism matter; interpret the absolute value and the trend
• Blood culture - not routine if suspicion is low, essential if the child is febrile or there is septic concern

Radiographs. AP and frog-leg views are usually normal but may show a widened joint space (Waldenstrom's sign) or soft-tissue swelling. Their primary role is to exclude Perthes disease, fractures and tumours.

MRI. Reserved for difficult cases, where it can show bone marrow oedema (osteomyelitis) or pyomyositis. The pair below compares transient synovitis with a septic hip.

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What it shows. Ultrasound is the most sensitive test for a hip effusion: an anechoic-to-hypoechoic collection distending the anterior joint recess, seen as a convex (bulging) anterior capsule lifted off the femoral neck. The usual threshold is a capsule-to-bone distance greater than about 5 mm, or a side-to-side difference of 2 mm or more compared with the asymptomatic hip.

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What it is for. It confirms the irritable hip, quantifies the effusion and guides aspiration. Longitudinal thickness and length plus transverse width define the anterior-recess effusion, and measurement standardises follow-up. A completely normal, symmetric scan makes a significant intra-articular hip problem unlikely.

What it cannot do. Ultrasound cannot distinguish septic arthritis from transient synovitis: both produce an effusion and the fluid looks similar. In Zamzam's series it was about 86% sensitive and 90% specific for a septic hip but was explicitly not safe to differentiate the two, and outcomes were worse when a false-negative scan delayed treatment. Only aspiration of the fluid (cell count, Gram stain, culture) tells them apart.

In practice. Use ultrasound to answer "is there an effusion, and where do I put the needle?", not "is this infected?". Never let a "just an effusion / looks like synovitis" scan reassure you out of aspirating a high-risk hip.

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Perthes disease. Legg-Calvé-Perthes disease is the main differential for a limping child in this age group (4-8 years).

When it does not settle. If symptoms persist beyond 2 weeks, it is not transient synovitis. Repeat the radiograph and look for early Perthes changes (crescent sign).

The other serious causes. Maintain a high index of suspicion for each of these:

• Juvenile idiopathic arthritis - chronic (over 6 weeks), with morning stiffness, swelling and warmth; ANA may be positive, and eye examination for uveitis is required
• Slipped capital femoral epiphysis - an older age group (10-16 years), typically an obese adolescent, with an external rotation deformity and obligatory external rotation on flexion
• Osteomyelitis - in the proximal femoral metaphysis; movement is painful but not as restricted as in a septic joint, inflammatory markers are high and MRI is diagnostic
• Leukaemia - night pain, constitutional symptoms (weight loss, bruising), generalised lymphadenopathy and an abnormal FBC (blasts, cytopenia)

The sequence. Management of the irritable hip runs in four steps.

1. Assess the child, not just the score. Appearance, fever history, ability to bear weight, pain severity, range, CRP trend and red flags determine pre-test concern. Escalation also weighs age, trajectory, ultrasound and the need for aspiration.
2. Use ultrasound to identify an effusion. It does not determine whether the fluid is sterile.
3. Aspirate when infection remains plausible or the result will change management, and send the full fluid panel (see Surgical Technique).
4. Treat infection urgently when it is supported or strongly suspected, with source control plus antibiotics. Do not let a low cell count or negative routine culture override purulence, sepsis or a deteriorating course.

The confirmed typical case. Treatment is relative rest and activity modification ("cuddle therapy") with analgesia when safe: NSAIDs (ibuprofen) are the mainstay, and paracetamol is also used. Parents are educated on the red flags (high fever, refusal to walk) and given explicit return precautions, and the child is reviewed promptly if fever, worsening pain, inability to bear weight or failure to improve develops. Follow-up ensures resolution.

Antibiotics. They are not indicated for a confidently diagnosed non-bacterial transient synovitis, and are withheld only once infection has been reasonably excluded. If infection remains plausible, obtain samples promptly and start appropriate therapy without avoidable delay in a septic or deteriorating child.

The scenario. A child with an irritable hip has already received antibiotics, for another presumed infection or for the hip itself. Recent antibiotics can lower culture yield and blunt the presentation of septic arthritis, so they make a low predictor count less reassuring. Routine joint culture may be negative despite ongoing infection or undrained pus.

What to do. Reassess the full trajectory, obtain blood cultures and sample the joint urgently for cell count, Gram stain, culture and appropriate molecular testing. In a septic or deteriorating child, do not delay effective antibiotics because aspiration or theatre access is pending.

The corollary. A culture-negative aspirate is not automatically transient synovitis. Interpret it alongside prior antibiotics, organism epidemiology, fluid appearance, molecular results and the response after source control.

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No surgical role. Transient synovitis is a medical condition, and there is no role for surgery in its treatment once confirmed. Surgery's role is diagnostic: hip aspiration to exclude septic arthritis, and arthrotomy only if septic arthritis is confirmed or strongly suspected (pus on aspiration).

Aspiration. The approach is anterior or medial, and ultrasound guidance is standard. It is done under conscious sedation, or general anaesthesia in toddlers, with a large-bore needle. Send the fluid for:

• Cell count (WCC) and differential (PMN%)
• Gram stain and culture
• Locally appropriate molecular tests
• Crystal analysis

Reading the fluid. Turbid or purulent fluid is septic until proven otherwise, and clear or straw-coloured fluid is likely transient synovitis. Always send fluid for culture even if it appears clear, as early septic arthritis may not be frankly purulent.

No single threshold. Synovial WCC and neutrophil percentage overlap between diagnoses: elevated counts support infection, but the overlap prevents a single cutoff. Purulence and positive microbiology strongly support infection, whereas a lower count, negative Gram stain or negative routine culture does not exclude early, partially treated or Kingella infection. Interpret the fluid as a pattern, not one WCC threshold.

Missed septic arthritis. Missing septic arthritis is the critical complication. It is mitigated by interpreting the Kocher predictors in context and aspirating whenever infection remains plausible.

Recurrence. Recurrent episodes are usually milder. Parents are educated and reassured that it is benign, and each episode is treated the same way: exclude sepsis, then NSAIDs.

Perthes disease. A causal link with transient synovitis is not established. Early Perthes can initially mimic synovitis, which is why symptoms that persist are radiographed.

Coxa magna. A rare, mild enlargement of the femoral head can occur from increased blood flow (hyperaemia) due to synovitis. It is asymptomatic, usually resolves or persists without consequence, and is not Perthes disease.

Function is excellent, with 100% return to sports and activities. There is no increased risk of osteoarthritis or avascular necrosis, which differentiates transient synovitis from Perthes disease.